The present invention relates to heat-sealable thermoplastic films, and especially to oriented thermoplastic films which have at least one heat-sealable layer comprising a terpolymer and which are distinguished from prior art films by improved qualities as regards transparency, surface hardness, and heat-sealability.
For many applications, e.g., for packaging foodstuffs, the industry demands films which have a low turbidity, preferably a high tensile strength in the longitudinal and/or transverse direction(s), low heat-sealing temperatures combined with a high heat-sealing strength, as well as a reduced permeability to gases and/or vapors.
Such a combination of desirable properties cannot be achieved by monofilms e.g., polyethylene, polypropylene, or polyester films.
Polypropylene or polyester films oriented by stretching, normally biaxial stretching by the known sheet stretching or tubular stretching processes, do have the desirable physical parameters as regards strength, viz., tensile strength, tear resistance, etc., as well as scratch resistance and low turbidity, but due to their high degree of orientation and crystallinity, caused by stretching, the stretched films are no longer capable of being heat-sealed.
Therefore, it has been suggested to provide thermoplastic films with a heat-sealable layer which may be applied to the support, e.g., from a solution or a dispersion by melt coating or by lamination.
From the great variety of known measures and techniques which are generally known to the man skilled in the art and need not be discussed in detail here, only German Auslegeschrift No. 1,184,071 shall be mentioned.
This published patent application discloses a process for the manufacture of an oriented, heat-sealable polypropylene film wherein a heat-sealable layer is applied to a polypropylene film base, either from a solution or dispersion, as in the case of polyvinylidene copolymers, or as a melt, as in the case of high-pressure polyethylene. Although such films are satisfactory as far as the desired strength and heat-sealing strength are concerned, they have the drawback that their turbidity is too high and that they are too sensitive to scratching. Such films may be used for some applications, where the additional qualities mentioned are of no importance, but they are unsuitable as packaging films, especially if hard goods, such as rice, noodles, potato chips, nuts and the like are to be packed, and these films will not be accepted by the industry.
From German Offenlegungsschrift No. 1,694,694, a heat-sealable laminate is known which consists of an oriented polypropylene film provided with at least one heat-sealable layer composed of a copolymer of 2 to 6 percent by weight of ethylene and 98 to 94 percent by weight of propylene. The above-mentioned drawbacks are avoided by this laminate. This composite film has a better heat-sealability than the above disclosed film, but it is not as clear and resistant to scratching as would be desirable. For this reason, this film is not particularly suitable for use on modern, vertical-type molding, filling and packaging machines, because the film surface will be scratched even during packaging by the goods to be filled in and also by the machine itself, e.g., when the film is drawn over the molding shoulders. Therefore, as a result of this fact, in combination with the inherent turbidity of the film, the package does not present the degree of optical clarity preferred by the user.
German Offenlegungsschrift No. 2,460,597 discloses a multilayer film which comprises an .alpha.-olefin substrate which is provided on at least one surface with a heat-sealable layer of a copolymer of polypropylene and an .alpha.-olefin with 4 to 10 carbon atoms in the molecule. The proportion of .alpha.-olefin in the heat-sealable layer is between 5 and 20 percent by weight. By incorporating such a high proportion of a comonomer (the preferred range being 10 to 15 percent by weight of 1-butene) the scratch resistance of the laminate is said to be improved over that of other films, e.g., films with other heat-sealable layers, and its adhesion to the sealing jaws is said to be reduced. On the other hand, the temperature at which the material begins to be heat-sealable is raised in an undesirable manner, which means that its heat-sealing range is narrowed. Such a restriction of the heat-sealing range will not be tolerated by the user, however, because he prefers to operate at sealing temperatures which are as low as possible, i.e., with as little shrinkage of the film as possible and with a minimum expenditure of energy. Furthermore, despite a high comonomer proportion, the necessary strength of the heat-sealing bond is frequently not achieved when lower temperatures within the sealing range are applied. Moreover, the resulting film is too turbid.